The temperature of an object may be measured by measuring the amount of infrared radiation emitted by heat sources. The emission of infrared (IR) radiation is higher with increase of source temperature. In traditional IR temperature sensors, the sensor is provided with a shielding cap which filters infrared radiation for transmission, blocking the rest. For instance, visible light may be blocked by a silicon filter, which lets substantially solely IR radiation pass. The IR radiation may be emitted by an object of interest, which is a heat source.
The shielding cap is typically thermally isolated from the sensing element, for instance by leaving a spatial gap of air or vacuum between the cap and the sensing element. The IR radiation is then collected by the sensing element, which heats up due to the collected energy. This sensing element may for example be a radiation absorbing membrane. The heating results in a temperature change. This temperature change is detected, for example by the hot contact of a thermocouple. The difference of temperature between the sensing element and a reference produces a readable electrical voltage signal which depends on the amount of IR radiation received from the object of interest (and hence, of the temperature of the object of interest). In the case of thermocouples, the reference is provided by a cold contact. The difference of temperature between the hot contact (the sensing element) and cold contact (the reference) creates a small voltage, which can be read as a measurement value for the amount of heat, emitted from the heat source, entering the detector. Hence the measurement value of the voltage is a value representative for the temperature of the heat source.
Normally the hot contact is attached to a membrane which absorbs the infrared radiation and the cold contact is attached to the bulk matrix of the device. Several factors contribute to errors in the measurement. The sensing element may receive parasitic signals from sources other than the object of interest, for instance from the detector housing. The increase of the reference temperature may also affect the measurement. In case of thermocouples, the bulk reference temperature may increase (for instance, due to environmental conditions) and create an offset, thus diminishing the accuracy of the measurement. Document U.S. Pat. No. 6,236,046 shows a detector comprising a first substrate, an active primary sensing element and a cap with a window allowing IR radiation to reach the active sensing element. The sensing element comprises a thermocouple with its cold junction attached to the first reference substrate. An additional passive secondary detector is allowed to receive IR radiation from the first reference substrate, so the offset is compensated by subtracting the signal of the additional detector from the signal of the active sensing element.
However, in practice, the parasitic contributions and the offset are not completely eliminated. For instance, the signals of the active and passive detectors may not compensate optimally due to differences in the characteristic and construction of the sensing elements. This may result in an offset value existing even when no object of interest (no heat source) is present.